The present invention relates to voltage protection, and more particularly to a voltage monitoring system for control and protection, for example, of capacitors or other equipment connected between high voltage terminals.
High voltage banks of capacitors comprising a plurality of parallel-connected capacitor groups in series are well known for power factor correction on alternating current transmission and distribution circuits. They are utilized also in high voltage harmonic filters associated with power rectifiers and inverters in high voltage direct current systems of power transmission.
When a large amount of capacitive reactance is required in a high voltage system, it is standard practice to connect a large number of relatively small capacitors of standard KVA and voltage rating in parallel groups each comprising a plurality of capacitors in parallel circuit relation, and to connect a plurality of such parallel groups in series between high voltage terminals. For any selected capacitor rating, the number of series-connected capacitor groups is determined by the voltage to be imposed across the bank. For example, such a bank may comprise ten capacitors connected between paralleling buses which define each capacitor group with ten such parallel-connected groups in series between line voltage and ground potential in each leg of a wye-connected, three phase bank of capacitors. On a 230 kilovolt system, each such leg may be exposed to a repetitive peak voltage of 200,000 volts or 20,000 volts across the parallel-connected group of capacitors at each voltage level.
In such capacitor banks, it is usual to provide an individual fuse in series with each capacitor between paralleling buses at each voltage level. If any one fuse in a group of paralleled capacitors operates to disconnect a capacitor the impedance of that paralleled group increases. Total voltage across the bank then no longer divides equally among the series-connected groups, but increases on the group containing the faulted capacitor or capacitors. Standard capacitors are designed for continuous operation at 110% of rated voltage. If overvoltage of more than 10% occurs on any group of parallel-connected capacitors, as due to removal of one or more from the group by fuse operation, the remaining capacitors in the group are endangered.
The magnitude of overvoltage imposed upon any capacitor group depends of course upon the number of paralleled capacitors in the group. Desirably each group comprises a sufficient number of capacitors such that the voltage rise caused by removal of one capacitor is no greater than 10%. It is desirable however to provide protective means for ensuring that no greater overvoltage is imposed upon the remaining capacitors, and many such arrangements have been devised.
In order to detect overvoltage on parallel groups of capacitors in a series-connected bank, it is the usual practice to utilize unbalance of voltage between normally equipotential points on two or more capacitor banks in parallel between high voltage terminals. To provide such differential voltage detection, of course, a capacitor installation must be divided into at least two banks (so that the capacitors at any one voltage level are in several separate groups), or else a separate reference voltage divider must be connected across the high voltage terminals.
It is not always desirable to divide an installation into several banks of small parallel groups. Each group should contain sufficient capacitors that overvoltage due to removal of a single capacitor will be limited to no more than 10%. Also, each group should contain sufficient capacitors such that the fault current through any short-circuited capacitor will clear the associated fuse before the casing of the faulted capacitor ruptures. Even if a separate reference voltage divider is connected in parallel with a single capacitor bank, detection by voltage unbalance is subject to faulty operation as a result of external system disturbance. Moreover, the calibration of a voltage unbalance detector is adversely affected by the presence of harmonic voltages. In harmonic filters, therefore, it is especially desirable to avoid detection arrangements responsive to voltage unbalance.
It is an object of the present invention to provide improved means for detecting overvoltages of a shunt capacitor bank which comprises a single series string of parallel-connected groups of capacitors.
It is a more general object of the present invention to provide an improved overvoltage monitor for control and protection of capacitors or other equipment connected across high voltage terminals.